Mineral Rights, Net - Mineral Rights (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	$ 815,818,000		$ 815,818,000		$ 820,170,000
Accumulated Depletion	(373,364,000)		(373,364,000)		(359,797,000)
Net Book Value	442,454,000		442,454,000		460,373,000
Asset impairments	57,000	$ 934,000	4,116,000	$ 133,217,000
Coal Mineral Rights
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	781,363,000		781,363,000		785,623,000
Accumulated Depletion	(359,683,000)		(359,683,000)		(346,773,000)
Net Book Value	421,680,000		421,680,000		438,850,000
Asset impairments			4,100,000	$ 133,200,000
Aggregates Mineral Rights
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	8,952,000		8,952,000		9,039,000
Accumulated Depletion	(3,084,000)		(3,084,000)		(2,819,000)
Net Book Value	5,868,000		5,868,000		6,220,000
Oil And Gas Mineral Rights
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	12,354,000		12,354,000		12,354,000
Accumulated Depletion	(8,985,000)		(8,985,000)		(8,593,000)
Net Book Value	3,369,000		3,369,000		3,761,000
Other Mineral Rights
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	13,149,000		13,149,000		13,154,000
Accumulated Depletion	(1,612,000)		(1,612,000)		(1,612,000)
Net Book Value	$ 11,537,000		$ 11,537,000		$ 11,542,000